To use the tunnel, simply drive your truck onto the train by going down the ramp of the open barriers. Your truck does not need to be fully on the train, just the cab over the icon.
The Channel Tunnel is located on the A20 in the UK, and on the A16 in France.
A ride through the Channel Tunnel costs €300/£240 and takes 35 minutes. The crossing time is accurate in both the game and real life and is the fastest way to transport vehicles from the UK to France or backward.